it won't need an additive, Alloy heads already are fine for unleaded fuel.
91 is crap, i don't even use it in the lawnmower.. you should try a full tank of each and see if you get better economy and you will probably feel the difference of the 95. i suspect you'll get better fuel economy from 95..
i ran Penrite HPR30 in mine, but the penrite app i have lists HPR15 so will depend how worn it is(Shell had an oil for older engines that used to shut up noisy lifters a bit more, was 25/50 i think)
as for oil and plugs, i used NGK ZGR6B11 in carby models(on Lpgas), i assume it's same for EFi?
Chis(Mr Polson) above works for Repco, and runs an efi crossflow also so he'd be worth taking advice from in a car that is standard.. if it was reconditioned, or extra sludgy then you'd consider asking an engine reconditioner what they'd suggest, but a worn out rattly crossflow doesn't need life support.. they are pretty hard to kill unless thrashed hard(even then the auto will probably die first )